The invention relates to a locking device that can be made as a nut or bolt. More particularly, though not exclusively, the invention relates to a locking device made as a nut or bolt for a vehicle wheel.
Locking devices made as nuts or bolts are known in the art. Such locking devices usually comprise a cylindrical body having either a longitudinal threaded bore, when the device is made as a nut, or a longitudinal threaded stem, when the device is made as a bolt. The bore can be a blind bore or a through bore, depending on the applications. Still according to prior art, the cylindrical body is further provided with an engaging portion for an actuating wrench, which allows to rotate the device. The engaging portion usually has a hexagonal cross-section and can therefore be engaged by a standard wrench of a fixed type with hexagonal cross-section. The wrench may possibly be mounted on a screwing device, for instance a pneumatic one. The wrench is guided in its engagement configuration by the engaging portion itself, which therefore also acts as a guide portion. When the device is in use, i.e. when it is engaged with a threaded stud or in a threaded seat, depending on whether the device is a nut or a bolt, respectively, the engaging portion usually remains visible from the outside, as it has to be accessible for the actuating wrench. For example, when the nut or bolt is engaged on or respectively in a vehicle wheel hub, the engaging portion remains visible on the outside of the wheel rim.
In some applications, for example in the automotive field, the need exists to make the engaging portion of the locking device aesthetically pleasing. This result can be obtained by means of treatments of different kind that are applied either to the whole body of the device or only to the engaging portion. For instance, the engaging portion of the device, when made of steel, can be subjected to a chromium plating treatment. Alternatively, the engaging portion can be varnished. As a further alternative, the engaging portion can be coated with a metal or plastic material. In this last instance, the coating can either adhere to the outer surface of the engaging portion or consist of a hollow cap which is press-fitted with interference onto the engaging portion. In some cases it is known to apply, between the cap and the engaging portion, a curable adhesive, which significantly improves the fixing of the two components. Fixing of the cap to the engaging portion must indeed be such as to prevent the cap from becoming detached from the device body as a consequence of the applied moment when a rotation torque is applied by the actuating wrench. GB 2378740 describes an example of this kind of arrangement. According to prior art, the cap can be made of various materials, for instance aluminum or chrome-plated steel.
One of the drawbacks of the known devices provided with caps results from the tendency of the cap to become detached from the device body, especially when the device must be thoroughly screwed with the locking torque prescribed by the manufacturing company of a vehicle. During locking and unlocking of the device, for instance when the device is applied to a vehicle wheel, the force applied to the cap tends to rotate the cap relative to the outer surface of the engaging portion. Notwithstanding the press-fitting and the adhesive employed by some manufacturers, this drawback is not entirely eliminated. Furthermore, the force applied to the cap by means of the actuating wrench often causes deterioration of the aesthetical features of the cap. For example, in the case of a chrome-plated cap, it often happens that the cap becomes scratched or even cracked when the torque necessary for locking the device according to the established safety parameters is applied.
In other applications, the engaging portion serves solely as guide portion and in the device body there is provided an engaging groove for an actuating wrench provided with a projection complementary to the groove. The actuating wrench is guided to engage into the groove by the guide portion. This is the case of the so-called anti-theft devices. These devices are especially widespread in the automotive field, where they are used against theft of wheel rims. The guide portion usually consists of a bushing which is idly rotatable relative to the device body. The engaging groove for the wrench is usually provided either in a radial portion of the device located at the base of the bushing, or at the center of the bushing. In the first instance the bushing is usually a blind bushing, whereas in the second instance it comprises an open face for allowing access to the groove. Examples of anti-theft devices of the described kind can be found in EP2469106 (A1). The bushing is usually cylindrical and has a smooth outer surface. There is, however, the need to make the bushing with a different kind of outer surface, for instance with a hexagonal cross-section. In some applications, especially in the automotive field, it is indeed necessary, for aesthetical reasons, that the device is outwardly identical to a conventional nut or bolt, in particular of a hexagonal type, mounted on the same wheel.
A first object of the invention is to provide a locking device that has a cap covering the guide and engaging portion for the actuating wrench and does not exhibit the drawbacks of the prior art. Another object of the invention is to provide a device of the aforementioned type that can be made either as a nut or as a bolt. A further object of the invention is to provide a locking device that is aesthetically pleasing and maintains its aesthetical features even after heavy use. A not least object of the invention is to provide a nut or bolt that can be manufactured industrially at low cost. Another object of the invention is to provide an anti-theft locking device that attains the objects set forth above.
These and other objects are achieved by the locking device made as a nut or bolt as claimed in the appended claims.